BACTERIA IN THE SOIL 



163 



all the advantage is on the side of the parasite, and nothing 

 but loss on the side of the host. The distinction between 

 symbiosis and commesalism cannot be rigid, but between 

 these conditions which are advantageous to the partners and 

 parasitism, there is an obvious and radical difference. As- 

 sociation of organisms together for increase of virulence and 

 function should be distinguished from symbiosis, and mere 

 existence of two or more species 

 of bacteria in one medium is 

 not, of course, symbiosis. Most 

 frequently such a condition 

 would result in injury and the 

 subsequent death of the weaker 

 partner, an effect precisely op- 

 posite to that defined by this 

 term. 



The example of bacteriologi- 

 cal symbiosis with which we are 

 concerned here is that partner- 

 ship between bacteria and some 

 of the higher plants (Legumin- 

 oscb) for the purpose of fixing 

 nitrogen in the plant and in the 

 surrounding soil. 



The Nitrogen- fixing Bacteria, 

 the third group of micro-organ- 

 isms connected with the soil, 

 exist in groups and colonies 

 situated inside the nodules ap- 

 pearing, under certain circum- 

 stances, on the rootlets of the 

 pea, bean, and other Leguntin- 

 os(B. It was Hellriegel and 

 Wilfarth who first pointed out that, although the higher 

 chlorophyllous plants could not directly obtain or utilise free 



Rootlet of Pea with Nodules 



